Organic electroluminescent (EL) devices or organic light-emitting diodes (OLEDs) are electronic devices that emit light in response to an applied potential. The structure of an OLED includes, in sequence, an anode, an organic EL unit, and a cathode. The organic EL unit disposed between the anode and the cathode is commonly comprised of an organic hole-transporting layer (HTL) and an organic electron-transporting layer (ETL). Holes and electrons recombine and emit light in the ETL near the interface of HTL/ETL. Tang et al., “Organic Electroluminescent Diodes”, Applied Physics Letters, 51, 913 (1987), and commonly assigned U.S. Pat. No. 4,769,292 demonstrated highly efficient OLEDs using such a layer structure. Since then, numerous OLEDs with alternative layer structures have been disclosed. For example, there are three layer OLEDs that contain an organic light-emitting layer (LEL) between the HTL and the ETL, such as that disclosed by Adachi et al., “Electroluminescence in Organic Films with Three-Layer Structure”, Japanese Journal of Applied Physics, 27, L269 (1988), and by Tang et al., “Electroluminescence of Doped Organic Thin Films”, Journal of Applied Physics, 65, 3610 (1989). The LEL commonly includes a host material doped with a guest material wherein the layer structures are denoted as HTL/LEL/ETL. Further, there are other multilayer OLEDs that contain more functional layers in the devices. At the same time, many kinds of EL materials are also synthesized and used in OLEDs. These new structures and new materials have further resulted in improved device performance.
An OLED is actually a current driven device. Its luminance is proportional to current density, but its lifetime is inversely proportional to current density. In order to achieve high brightness, an OLED has to be operated at a relatively high current density, but this will result in a short lifetime. Thus, it is critical to improve the luminous efficiency of an OLED while operating at the lowest possible current density consistent with the intended luminance requirement to increase the operational lifetime.
In order to dramatically improve luminous efficiency and to increase lifetime in OLED, a tandem OLED (or stacked OLED, or cascaded OLED) structure, which is fabricated by stacking several individual OLEDs vertically and driven by only a single power source, has been fabricated (see U.S. Pat. Nos. 6,337,492, 6,107,734, 6,717,358, U.S. Patent Publication Nos. 2003/0170491 A1, 2003/0189401 A1, and JP Patent Publication No. 2003045676A). In a tandem OLED having a number of N(N>1) EL units, the luminous efficiency can be N times as high as that of a conventional OLED containing only one EL unit (of course, the drive voltage can also be N times as high as that of the conventional OLED). Therefore, in one aspect to achieve long lifetime, the tandem OLED needs only about 1/N of the current density used in the conventional OLED to obtain the same luminance while the lifetime of the tandem OLED will be about N times that of the conventional OLED. In the other aspect to achieve high luminance, the tandem OLED needs only the same current density used in the conventional OLED to obtain a luminance N times as high as that of the conventional OLED while maintaining about the same lifetime.
Although tandem OLEDs have many advantages, one disadvantage is the increased drive voltage. In many electronic systems, e.g., in some active matrix designs, the available voltage is limited. Thus, there is a need to reduce the voltage necessary to drive tandem OLEDs. One way to lower driving voltage in a tandem OLED is to provide an electron-injecting layer (EIL), which typically includes an electron-transporting material doped with an n-type dopant such as a low-work function metal. For example, see U.S. Pat. Nos. 6,013,384, 6,509,109, 6,566,807, and 6,589,673. The EIL is provided between the cathode and the light-emitting layer, and typically in contact with the cathode. However, the metallic dopant can cause excited-state quenching and lower the luminance efficiency. This occurs if the EIL is directly on the light-emitting layer, or if the electron-transporting material selected for the EIL does not effectively bind the metal dopant, thus permitting diffusion of the metal into the light-emitting layer. Such a situation also shortens the lifetime of the OLED device. The problem of lowering voltage is not just limited to tandem OLED devices.
In addition to continued need to provide OLEDs having improved lifetime and efficiency, it is desirable to improve manufacturability of OLED devices. One way to simplify manufacturing is to limit shadow mask patterning and instead provide a white or broadband light-emitting OLED with color filters. For lowest power consumption, it is often advantageous for the chromaticity of the white light-emitting OLED to be close to CIE D65, i.e., CIE x=0.31 and CIE y=0.33. This is particularly the case for so-called RGBW displays having red, green, blue, and white pixels. However, many white or broadband OLED devices have multiple emissive layers, which results in higher drive voltage. Thus, there is a need to reduce the drive voltage and still achieve a desirable white point.